Method and apparatus for promoting biaxially uniform particle size distribution in an airstream containing coal particles of mixed sizes

ABSTRACT

An apparatus for promoting biaxially uniform distribution of coal particle size in an airstream supplying combustible mixture to a firebox by way of a header having a quadrangular arrangement of feed pipes. A conduit or duct which receives the coal air mix from a bowl mill pulverizer uses first and second single axis mixer elements in series, the operating or mixing axes of the two elements being disposed orthogonal to one another and at right angles to the air flow direction in the vicinity of the mixer elements. Deflector bars are disposed immediately upstream of the mixer elements.

FIELD OF THE INVENTION

[0001] This invention relates to the conveyance of pulverized coal from a pulverizer to a firebox and more particularly to the promotion of uniform particle size distribution in the airstream so that the firebox receives balanced charges through the several branches of an inlet manifold.

BACKGROUND OF THE INVENTION

[0002] Electrical power generation plants with coal-fired boilers use pulverizers to reduce raw coal to a relatively fine particulate air-coal mixture. This mixture is conveyed to a combustion chamber or firebox where it is ignited. As shown in FIG. 1 of U.S. Pat. No. 6,260,490 the airstream may be divided into, for example, four separate streams by a branched manifold upstream of the firebox. The four streams enter the firebox at uniformly spaced locations, in this case at the four corners. Maintaining the desired shape, intensity and temperature of the fireball requires that the charges delivered through the various branches of the manifold be properly balanced. In the case of a firebox having a square cross section and four inlets with uniform spacing, a balanced charge equates to four airstreams with substantially equal coal loads.

[0003] It is well known that the coal particles emerging from classifier type pulverizers are of mixed sizes and that the particle sizes tend to distribute themselves in a non uniform way throughout the airstream, largely as a result of the inertial effects of passing through elbows and intersections in the duct work between the pulverizer and the combustion chamber. The non-uniform particle size distribution can produce substantial differences in the charges or coal loads delivered to the various inlets to the firebox. As indicated above, this is disadvantageous.

SUMMARY OF THE INVENTION

[0004] The primary objective of the present invention is to promote biaxial uniformity in the particle size distribution of coal or other particulate matter carried by an airstream through a duct or conduit wherein the particles of particulate matter are of mixed sizes. When used to supply pulverized coal to a firebox through a delivery system having a branched manifold serving multiple firebox inlets, the invention tends to equalize the coal loads or charges delivered to the firebox.

[0005] According to the apparatus aspect of my invention, I provide an airflow duct having an inlet and an outlet, the inlet being suitable for connection to a source of particles of mixed sizes carried in an airstream. In the illustrative application described herein a coal pulverizer is the particulate source. I further provide a first single-axis mixer element in the duct at a first location between the inlet and outlet for cross-mixing particles which are size distributed along a first axis extending across the airstream. I further provide a second single-axis mixture element disposed in the duct at a second location downstream of the first mixer element for cross-mixing particles which are size distributed along a second cross-axis which is orthogonal to the first cross-axis. As necessary, I provide adjustable deflector bars upstream of one or both of the mixer elements. The result is a particulate/air mix emerging from the outlet of the duct which is substantially more uniform than the particle size distribution at the inlet. When connected to a quadrangular distributor manifold used to feed a firebox for a boiler, the result is a more balanced feed of the combustible mixture to the firebox and a better controlled fireball in the box.

[0006] In the illustrative example used herein wherein an elbow in the duct lies upstream of the first element, I place a deflector bar in the duct upstream of the first mixer element to deflect and redirect the inertially influenced flow back toward the center of the duct. Also in the illustrative example, I use “riffle box” cross-mixer elements of a generally triangular shape comprising a plurality of stacked flow channels which are parallel to the cross-axis along which mixing occurs. While each of the stacked parallel flow channels in the element has an inlet extending fully across the duct, the channels are alternately arranged to direct the air/particulate flow diagonally left and right across the duct so as to emerge back into the duct on the left and right sides of the element. Since the second cross-mixer element, which may otherwise be similar or identical to the first cross-mixer element, is disposed orthogonally to the first cross-mixer element, I mix first along one cross-axis and then along a second cross-axis which is orthogonal to the first. The terms “left” and “right” are used herein in a relative sense and do not denote specific directions.

[0007] The biaxial mixing which is provided by the apparatus described above is particularly suitable for use in connection with a firebox manifold header having a quadrangular or square arrangement of four outlet pipes communicating with the four comers of a combustion chamber or firebox as opposed to a header having multiple outlet pipes which are linearly disposed relative to one another.

[0008] According to the method aspect of my invention, I promote the uniform distribution of coal particle sizes in a coal particle/airstream through a method comprising the steps of: (a) causing the non uniform size-distributed particle/air mix to enter a duct, (b) particle size mixing first along a first cross-axis, and thereafter (c) particle size mixing along a second cross-axis which is orthogonal to the first.

[0009] In the preferred form I thereafter cause the uniform particle/air mix to enter a header having a quadrangularly arranged plurality of outlet pipes.

[0010] Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:

[0012]FIG. 1 is a diagram of a coal pulverizing and conveying system including the combination of a pulverizer, flow duct, header and firebox;

[0013]FIG. 2 is an enlarged and more detailed drawing of the duct of the system of FIG. 1;

[0014]FIG. 3 shows the apparatus of FIG. 2 from the right side of FIG. 2;

[0015]FIG. 4 is a detailed view of one of the cross-mixer elements;

[0016]FIG. 5 is a first detailed view of a deflector bar;

[0017]FIG. 6 is a second detailed view of a deflector bar; and

[0018]FIG. 7 is a plan view of the header showing the quadrilateral arrangement of ducts or branches.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0019] Referring to FIG. 11 diagrammatically show the major components of a coal pulverizing and conveying system embodying my invention. The system comprises a pulverizer 10, sometimes called a “bowl mill;” which is preferably of the type described in detail in my U.S. Pat. No. 5,386,619 issued Feb. 7, 1995, the full disclosure of which is incorporated herein by reference. Another suitable pulverizer is described in U.S. Pat. No. 4,687,145 issued Aug. 18, 1987. The pulverizer 10 has an inlet 12 for raw coal which is fed by gravity and/or a combination of conveyors and gravity into the top of the pulverizer where it is distributed onto a rotatable grinding surface (not shown) and pulverized by rollers (not shown) which engage the grinding surface. The pulverizer has a “classifier” function wherein large particles are returned to the grinding surface while finer particles are caused to exit the pulverizer by an upwardly directed air stream. The resulting pulverized coal is caused to flow upwardly and outwardly through an outlet 14 into a feed pipe 43 connected to a duct 16 which may have a plurality of seams 18, 20, 22 and 23 where sections of the duct are joined to one another such as by welded or bolted flanges. The duct 16 is connected to and extends between the feed pipe 43 and a header 24 having outlet pipes 26, 28, 30 and 32 connected to the four corners of a combustion chamber or firebox 34 wherein the combustible coal air mixture is ignited and burned to create heat which is thereafter used to boil water to turn a turbine which drives an electrical generator in a well-known manner. The details of the steam-to-electricity conversion form no part of the present invention.

[0020] In accordance with my invention I promote the biaxially uniform distribution of the coal particle sizes in the airstream which enters the header 24 so as to promote uniformity in the coal/air mixture which flows through the pipes 26, 28, 30 and 32. The result is that all four corners of the firebox 34 are relatively equally fed and lean/rich disparities between the four corners of the firebox are reduced and/or eliminated.

[0021] Referring now to FIGS. 1-3, I place a first single axis mixer element 36 in the duct 16 between the pulverizer outlet 14 and the header 24. For purposes of discussion the seam 18 is taken as the inlet to the duct 16 and the seam 23 as the outlet. The first mixer element 36 is between the inlet and the outlet as defined above. The axis of operation of cross-mixer element 36 is such as to lie in the plane of the paper in the view of FIG. 1.

[0022] I place a second single axis mixer element 38 in the duct 16 downstream of the first mixer element 36 to promote particle size mixing and uniformity across a second axis which is orthogonal to the operating axis of the element 36. Using two orthogonally-oriented series connected single axis mixers, I promote biaxially particle size distribution in the air coal particle/airflow mixture exiting the duct 16 and entering the header 24. The header 24 is better shown in FIG. 7 to comprise a quadrangular arrangement of inlet pipes which feed the four comers of the firebox 34. This is to be distinguished from the in-line arrangement shown at 40, 42, 44, and 46 of U.S. Pat. No. 6,260,490.

[0023] Because the duct 16 is shown to comprise an elbow 42 immediately upstream of the mixing element 36, I place a deflector bar 40 which is adjustable in angular orientation at the outside extremity of only the downstream portion of the elbow 42; i.e., in the duct 16 just after the elbow and just before the airstream enters the mixer element 36. If the mixer 36 were fed by a straight inlet pipe or duct, I would place deflector bars on both sides of the duct; i.e., on the left and right sides as seen in FIG. 1.

[0024] Referring again to FIGS. 2-5, further details of the illustrative embodiment of my invention will be described. In this example the cross-section of the feed pipe 43 upstream of the seam 18 is circular and the cross-sections of the elbow 42 and duct 16 downstream of the seam 18 are square or rectangular. The deflector bar 40 is shown in FIGS. 5 and 6 to comprise a shaft 46 welded to a long steel bar 48 having teeth 50 and 51 formed in the edge opposite the shaft 46 so as to extend into the airstream. Additional plates 52, 53 and 54 are welded to the inside surface of the bar 48 to aid in the deflection of air and entrained particulates back toward the center of the duct 42 before it enters the mixer 36. The angular position of the shaft 46 is adjusted to create the desired deflection effect and the shaft 46 is thereafter suitably clamped or bolted into position. The net effect is to balance the flow between the left and right sides of duct 16.

[0025] In the illustrated embodiment I also place a number of diffuser bars 56 and 58 in the elbow 42 upstream of the deflector bar 40 in the elbow 42. All of the components 40, 50, 56 and 58 create turbulence and a mixing effect in the particulate/air mix in the duct 16.

[0026] Continuing the detailed description, the particulate/air mix now flows into the first single axis cross-mixer element 36. This cross-mixer element, also known as a “riffle box,” is best shown in FIG. 4 to comprise a welded plate-like structure defining a plurality of flow channels having parallel inlets 37 a and 37 b which extend across the duct 16 parallel to a first cross-axis which lies in the plane of the paper in FIG. 2. In this illustration there are two inlets 37 a and two inlets 37 b. Moreover, the inlets 37 a and 37 b are stacked in alternating order. The channels 37 a and 37 b, although they each collect coal particles through inlets which extend across the duct, are diagonally alternately arranged so as to direct the collected particles to the right and left, respectively. Specifically, the top channel with inlet 37 a is defined in part by an internal rib 39 which extend to a right side outlet 41. The next inlet 37 b opens to a channel having an internal rib 33 which extends to a left side outlet 35. The next lower inlet 37 a goes to right side outlet 47 and so on to the bottom of the stack. There are typically equal numbers of left and right-directing channels, the net effect being to evenly distribute coal particles of mixed sizes across the duct 16.

[0027] After emerging from the mixer 36, the coal particle/airstream encounters a second set of deflector bars 60 and 61 similar in construction to deflector bar 40 but mounted with the longitudinal axes thereof orthogonal to the longitudinal axis of shaft 46. Each deflector bar 60 and 61 is welded to a mounting shaft like shaft 46 and has both edge teeth and deflectors as illustrated in FIGS. 5 and 6. In the structure shown, the deflectors 60 and 61 and their mounting shafts are divided at the centers and secured at the centers to a partition 64 in the duct 16. In effect, each deflector bar 60 and 61 has two longitudinal partitions and those portions may be independently adjusted in angle to fine tune the left-right balance of flow out of mixer 36 and into mixer 38.

[0028] Downstream of the deflector bars 60 and 61 is a second single axis mixer element 38 of a construction which is generally similar to the construction of the single axis mixer element 36. However, the mixer element 38 is arranged with its cross-axis orthogonal to the mixing axis of the element 36; i.e., the mixing axis or cross-axis of the element 38 lies in the plane of the paper as shown in FIG. 3.

[0029] Downstream of the element 38 is the header box 24 which, as best shown in FIG. 7, has a quadrangular or square arrangement of feed pipes 26, 28, 30 and 32 as opposed to the in-line arrangement of U.S Pat. No. 6,260,490. This square arrangement is favored because the particle distribution in the twice-mixed airstream is substantially uniform in a bidirectional sense such that relatively equal charges enter the four feed pipes 26, 28, 30 and 32.

[0030] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law. 

What is claimed is:
 1. Apparatus for promoting biaxially uniform particle size distribution in an airstream containing particles of mixed sizes comprising: an air-flow duct having an inlet and an outlet and defining an air-flow direction; a first mixer element disposed in the duct at a first location between the inlet and outlet for mixing particles which are size distributed along a first axis extending across the air-flow; and a second mixer element disposed in the duct at a second location downstream of the first mixer element for mixing particles which are size distributed along a second axis extending across the air-flow direction; the first cross-axis being orthogonal to the second cross-axis whereby the particle size distribution at the outlet is more uniform than the particle size distribution at the inlet of the duct.
 2. Apparatus as defined in claim 1 wherein the first mixer element comprises a plurality of stacked parallel flow channels which collect air and particles across flow channel inlets extending fully across the duct and parallel to the first cross-axis, said stacked flow channels being arranged to alternately oppositely direct the collective air and particles to opposite sides of the duct measured along the first cross-axis.
 3. Apparatus as defined in claim 1 wherein the second mixer element comprises a plurality of stacked parallel flow channels which collect air and particles across flow channel inlets extending fully across the duct and parallel to the second cross-axis, said stacked flow channels alternately oppositely directing the collective air and particles to opposite sides of the duct measured along the second cross-axis.
 4. Apparatus as defined in claim 1 further including a deflector bar disposed in the duct upstream of the first mixer element and having a longitudinal axis which is orthogonal to the first cross-axis.
 5. Apparatus as defined in claim 4 wherein the deflector bar is angularly adjustable in position.
 6. Apparatus as defined in claim 5 wherein the deflector bar has teeth formed thereon.
 7. Apparatus as defined in claim 4 further comprising its second deflector bar disposed in the duct between the first and second mixer elements and having a longitudinal axis which is orthogonal to the second cross-axis.
 8. Apparatus as defined in claim 7 wherein the second deflector bar is angularly adjustable about the longitudinal axis thereof.
 9. Apparatus as defined in claim 8 wherein the second deflector bar has teeth formed thereon.
 10. Apparatus as defined in claim 1 further comprising a header means attached to the outlet of the duct for receiving air and particles flowing therefrom. Said header means comprising four quadrangularly arranged outlet pipes adapted to convey a coal particle/air mixture to a combustion chamber.
 11. A method of promoting biaxially uniform particle size distribution in a coal particle/air stream wherein the coal particles are of mixed sizes comprising the steps of: a. causing the coal particle/air mix to enter a duct; b. mixing the particle sizes across a first axis extending across the air flow direction; c. thereafter mixing the particle/airstream across a second axis extending across the air-flow direction wherein the second cross-axis is orthogonal to the first cross-axis; and d. thereafter directing the twice-mixed coal particle/air mixture to a combustion chamber.
 12. The method of claim 11 wherein the step of directing the twice-mixed mixture is carried out using a header structure with four quadrangularly arranged feed pipe outlets. 